1. Field of the Invention
The present disclosure relates to a wide viewing angle liquid crystal display device (or “LCD”) working in normally white mode. Especially, the present disclosure relates to an LCD operating in normally white mode in which a nematic phase liquid crystal layer is driven by In-Plane Switching method for having a wide view angle feature.
2. Discussion of the Related Art
The most used LCD shows the picture data by modulating the luminescence of the light incident from the backlight unit by controlling the electric field applied to the liquid crystal layer. Generally, the liquid crystal display panel comprises a plurality of liquid crystal cells disposed in matrix type and an upper polarizer and a lower polarizer which are disposed at upper side and lower side of the liquid crystal cells, respectively, and in manner that the light transparent axes of them are perpendicularly disposed each other. The liquid crystal cells have the liquid crystal materials having the dielectic anisotropy and optical anisotropy. The liquid crystal panel further comprises a pixel electrode and a common electrode to form an electric field for driving the liquid crystal material. The pixel electrode can be connected to a switching element such as a thin film transistor (or “TFT”). The LCD device driving the liquid crystal material using the TFT is called as TFT-LCD. For the liquid crystal material, the nematic phase liquid crystal material is mostly used. According to the method for driving the liquid crystal cells, there are a vertical driving type and a horizontal driving type.
The vertical driving type TFT-LCD is also called as TN (Twisted Nematic) mode TFT-LCD of which structure is as shown in FIG. 1. FIG. 1 is a perspective view illustrating the TN mode TFT-LCD operating in normally white mode according to the related art. FIGS. 2A and 2B briefly show the operating principle of the TN mode TFT-LCD according to the FIG. 1.
Referring to FIG. 1, the TN mode TFT-LCD 10 comprises an upper substrate 11 and lower substrate 31 which are disposed in facing each other, and liquid crystal cell of liquid crystal molecules 41 disposed between the substrates 11 and 31. On the lower substrate 31, a plurality of scan line 33 and a plurality of data line 35 are crossly disposed to define a plurality of pixel arrayed in matrix type. The TFT 37 is formed where the scan line 33 and the data line 35 is crossed. The TFT 37 is electrically connected to the pixel electrode 39 supplying a positive electric voltage to the liquid crystal molecules 41 of the liquid crystal cell.
On the upper substrate 11, black matrix 13 is disposed in the matix pattern at the position corresponding to the scan line 33 and the data line 35 of the lower substrate for defining the pixel. At the pixel, color filter 15 is formed in manner that the color filters 15 representing R(red) color, G(green) color and B(blue) color are sequentially arrayed. On the layer of the color filter 15, a common electrode 17 is formed for supplying a negative voltage to the liquid crystal molecules 41.
Furthermore, an upper polarizer 21 and a lower polarizer 23 are disposed on the upper outside of the upper substrate 11 and the lower outside of the lower substrate 31, respectively. The polarizers 21 and 23 make the incident light to be linearly polarized. Therefore, in order to get full black gray-scale perfectly, the upper polarizer 21 and the lower polarizer 23 should be disposed in manner that their polarizing axes are perpendicularly crossed each other. The liquid crystal molecules 41 are disposed in manner that liquid crystal molecules 41 are parallel but continuously twisted until the direction of the liquid crystal molecule near the upper substrate 11 and the direction of the liquid crystal molecule near the lower substrate 31 are crossed with 90 degrees.
Referring to FIGS. 2A and 2B, the operating conditions of the TN mode TFT-LCD 10 is explained. In FIGS. 2A and 2B, the detailed explain is focused on the upper polarizer 21, the lower polarizer 23 and the liquid crystal molecules 41 mainly. For the elements not shown in the FIGS. 2A and 2B, see FIG. 1.
The TN mode TFT-LCD 10 is operated in the normally white mode in which the white gray-scale is represented as the incident light through the lower polarizer 23 passes through the liquid crystal molecules 41 and the upper polarizer 21 when there is no electric field applied between the pixel electrode 39 and the common electrode 17. The incident light 43 from the light source (not shown) to the lower polarizer 23 is linearly polarized parallel to the polarizing axis of the lower polarizer 23 as incident light 43 passes through the lower polarizer 23. In FIG. 2A, the polarizing axis 3 of the lower polarizer 23 is set to 0° parallel to the x-axis of the rectangular coordinate system shown in the left corner, and the polarizing axis 1 of the upper polarizer 21 is set to 90° parallel to the y-axis. The phase of the light linearly polarized at 0° by passing through the lower polarizer 23 is delayed as passing through the twisted liquid crystal molecules 41 so that the axis of the linearly polarized light is changed to 90° parallel to the polarizing axis 1 of the upper polarizer 21. Therefore, because that the incident light 43 is linearly polarized by the lower polarizer 23, the polarization direction is changed by the twisted liquid crystal molecules 41 and then the light passes the upper polarizer 21, the LCD represents the full white gray-scale.
On the contrary, if a vertical electric field is formed between the pixel electrode 39 and the common electrode 17 as shown in FIG. 2B, the twisted structure of the liquid crystal molecules 41 will be broken by the dipole moment due to the vertical electric field. As a result, the liquid crystal molecules 41 are rearranged to the z-axis parallel with the direction of the applied electric field. In this case, the incident light 43 is also linearly polarized to 0° parallel to the x-axis by the lower polarizer 23, but this polarizing status is maintained as the light is passing through the liquid crystal molecules 41. Therefore, as the light passing through the liquid crystal molecules 41 has the polarizing axis perpendicular to the the polarizing axis 1 of the upper polarizer 21, it can not pass the upper polarizer 21. The LCD represents the full black gray-scale.
With these features, the TN mode TFT-LCD can be applied to the transparent display devices because it can be used as a transparent glass when there is no electric filed for driving the liquid crystal layer whilst it can be used as a display device when there is electric field for driving the liquid crystal layer. Furthermore, as the TN mode TFT-LCD can represents video information under the sun light with good quality, it can be applied to the reflective display device in which the backlight may not be needed, or to the semi-transparent display devices. However, due to the initial condition of the liquid crystal molecules, TN mode TFT-LCD has the narrow view angle property.
For the wide angle LCD device using the nematic phase liquid crystal material, there is an IPS mode TFT-LCD as shown in FIG. 3. FIG. 3 is a perspective view illustrating the IPS mode TFT-LCD operating in normally black mode according to the related art. FIGS. 4A and 4B briefly show the operating principle of the IPS mode TFT-LCD according to the FIG. 3.
Referring to FIG. 3, the IPS mode TFT-LCD 50, as the TN mode TFT-LCD 10, comprises an upper substrate 51 and a lower substrate 71 which are disposed in facing each other, and liquid crystal cell of liquid crystal molecules 81 disposed between the substrates 51 and 71. On the lower substrate 71, a plurality of scan line 73 and a plurality of data line 75 are crossly disposed to define a plurality of pixel arrayed in matrix type. The TFT 77 is formed where the scan line 73 and the data line 75 is crossed. The TFT 77 is electrically connected to the pixel electrode 79 supplying a positive electric voltage to the liquid crystal molecules 81 of the liquid crystal cell. In order to form a horizontal electric field to the liquid crystal molecules 81, the IPS mode TFT-LCD 50 includes common electrode 57 formed on the lower substrate 71 with being parallel to the pixel electrode 79.
On the upper substrate 51, black matrix 53 is disposed in the matix pattern at the position corresponding to the scan line 73 and the data line 75 of the lower substrate for defining the pixel. At the pixel, color filter 55 is formed in manner that the color filters 55 representing R(red) color, G(green) color and B(blue) color are sequentially arrayed.
Furthermore, an upper polarizer 61 and a lower polarizer 63 are disposed on the upper outside of the upper substrate 51 and the lower outside of the lower substrate 71, respectively. In order to get full black gray-scale perfectly, the upper polarizer 61 and the lower polarizer 63 should be disposed in manner that their polarizing axes are perpendicularly crossed each other. The liquid crystal molecules 81 disposed between the upper substrate 51 and the lower substrate 71 are arrayed with having an initial alignment direction. For example, the liquid crystal molecules 81 have the initial alignment parallel to the polarizing axis of the upper polarizer 61. Even though it is not shown in figures, the initial alignment condition of the liquid crystal molecules 81 can be set by disposing alignment layers inside surfaces of the upper substrate 51 and the lower substrate 71 which are contacting the liquid crystal molecules 81, and by forming a rubbing pattern on the alignment layers along with the initial alignment direction.
Referring to FIGS. 4A and 4B, the operating conditions of the IPS mode TFT-LCD 50 is explained. In FIGS. 4A and 4B, the detailed explain is focused on the upper polarizer 61, the lower polarizer 63 and the liquid crystal molecules 81 mainly. For the elements not shown in the FIGS. 4A and 4B, see FIG. 3.
The IPS mode TFT-LCD 50 is operated in the normally black mode in which the black gray-scale is represented as the incident light through the lower polarizer 63 passes through the liquid crystal molecules 81 but it cannot pass through the upper polarizer 61 when there is no electric field applied between the pixel electrode 79 and the common electrode 57. The incident light 83 from the light source (not shown) to the lower polarizer 63 is linearly polarized parallel to the polarizing axis 3 of the lower polarizer 63 as incident light 83 passes through the lower polarizer 63. In FIG. 4A, the polarizing axis 3 of the lower polarizer 63 is set to 0° parallel to the X-axis of the rectangular coordinate system shown in the left corner, and the polarizing axis 1 of the upper polarizer 61 is set to 90° parallel to the Y-axis. As the light linearly polarized at 0° by passing through the lower polarizer 63 passes the liquid crystal molecules 81 arrayed parallel to the polarizing axis 1 of the upper polarizer 63 through the short axis of the liquid crystal molecules 81, the polarized light does not have phase difference. That is, the polarized condition of the light 83 is maintained until the polarized light meets to the upper polarizer 61. Therefore, after the incident light 83 is linearly polarized by the lower polarizer 63, it cannot pass the upper polarizer 61, so that the LCD represents the full black gray-scale.
On the contrary, if a horizontal electric field is formed between the pixel electrode 79 and the common electrode 57 as shown in FIG. 4B, the liquid crystal molecules 81 will be rearranged to be parallel to the direction of the electric field by the dipole moment due to the horizontal electric field. In that case, the incident light 83 is also linearly polarized to 0° parallel to the X-axis by the lower polarizer 63, but the direction of the liquid crystal molecules 81 rearranged by the horizontal electric field is neither perpendicular nor parallel to the linearly polarized direction. As the linearly polarized light passes through the liquid crystal molecules 81, the linearly polarized light can be affected by the optical properties of the long axis and short axis. That is, the phases of the linearly polarized light will be changed. As a result, the light can pass the upper polarizer 61 and the LCD represents the full white gray-scale.
With these features, the liquid crystal molecules of the IPS mode TFT-LCD are driven on the In-plane condition, so that there is no phase different between the just front view direction of the LCD panel and the side view direction of the LCD panel. That is, the IPS mode TFT-LCD has wide view angle property, almost close to 180°. Furthermore, as the speed for driving the liquid crystal molecules is very fast, the IPS mode TFT-LCD is more acceptable to be applied to the TV monitor. However, in the most cases of the IPS mode TFT-LCD, the video data can be represented only when the power is on, and if the power is off, the display panel shows only black panel. Therefore, it is hard for the IPS mode TFT-LCD to be applied to the transparent display device or semi-transparent display device mostly used for the outdoor display device.
In order to meet the demand for various type display device, there have been many efforts for manufacturing the IPS mode TFT-LCD operating in normally white mode. One of typical example for that is shown in FIGS. 5A and 5B. FIGS. 5A and 5B briefly show the operating principle of the IPS mode TFT-LCD operating in normally white mode according to the related art.
Referring to FIG. 5A, the polarizing axis 3 of the lower polarizer 63 is set to 135°, the polarizing axis 1 of the upper polarizer 61 is set to 45°, and the initial alignment direction of the liquid crystal molecules 81 is set to 90° (or 0°). In this case, when there is no electric field for driving the liquid crystal molecules 81, the LCD can represent white gray-scale because the liquid crystal molecules 81 has 45° angle to both polarizing axes of the upper polarizer 61 and lower polarizer 63 (according to the same reason explained for FIG. 4B).
In addition, as shown in FIG. 5B, when the electric field is applied for driving the liquid crystal molecules 81, the liquid crystal molecules 81 are rearranged to be parallel to the direction of the electric field. Therefore, the incident light 83 is linearly polarized by the lower polarizer 63 and then it meets the liquid crystal molecules 81 which are either parallel or perpendicular to the direction of the linearly polarized light. As a result, the linearly polarized light does not have phase delay as it passes the liquid crystal molecules 81 and finally, the direction of the linearly polarized light will be maintained. For example, when the liquid crystal molecules 81 are rearranged to 45° parallel to the polarizing axis 1 of the upper polarizer 61 due to the horizontal electric field applied between the pixel electrode 79 and the common electrode 57, the incident light 83 is linearly polarized by the lower polarizer 63 and the polarized light pass the liquid crystal molecules 81 maintaining the polarized condition because the directions of the linearly polarized light and the liquid crystal molecules 81 are perpendicular (or parallel). Finally the polarized light cannot pass the upper polarizer 61. As a result, LCD represents fully black gray-scale (according to the same reason explained for FIG. 4A).
With these features, the IPS mode TFT-LCD operating in normally white mode cannot represent the perfectly full white gray-scale, but yellowish white gray-scale. This yellowish phenomenon is determined by the the cell gap (i.e. thickness of the liquid crystal cell) and the refractive index difference of the liquid crystal material.
That is, the light transmittence of the liquid crystal cell is denoted as the Equation 1. Here, Γ is denoted as the Equation 2.
                    T        =                              1            2                    ⁢                      sin            2                    ⁢          2          ⁢          α          ×                                    sin              2                        ⁡                          (                              Γ                2                            )                                                          Equation        ⁢                                  ⁢        1                                Γ        =                              2            ⁢                          π              ⁡                              (                                                      n                    e                                    -                                      n                    o                                                  )                                      ⁢                          d              λ                                =                                    2              ⁢              πΔ              ⁢                                                          ⁢              nd                        λ                                              Equation        ⁢                                  ⁢        2            
According to the Equations 1 and 2, the light transmittence is decided by the product of Δn and d. Here, Δn means the refractive index difference of the liquid crystal material, the difference between the refractive index in the long axis (ne) of the liquid crystal molecule and the refactive index in the short axis (no) of the liquid crystal molecule. FIG. 6 shows the difference between the refractive index in the long axis (ne) of the liquid crystal molecule and the refactive index in the short axis (no) of the liquid crystal molecule. As the liquid crystal molecule has long stick shape, Δn is always greater than 0. Therefore, according to the Equation 2, value of Γ should be greater than 0. Therefore, yellowish phenomenon is always occurred. Experimentally, in order that the light transmittance, T, meets white gray-scale without yellowish phenomenon, Δn should be 0.27˜0.32 (μm). However, in real-situation, it is almost impossible to meet this condition even though the cell gap is controlled or any other types of the liquid crystal materials are used.
In the related art, a method has been suggested to solve the yellowish phenomenon, as shown in FIG. 7, in which a half wave plate 5 (or “HWP”, or calles as ‘half wave retaring plate’) is inserted under the upper polarizer 61. For example, referring to FIG. 7, the polarizing axis 3 of the lower polarizer 63 is set to 0°, the polarizing axis 1 of the upper polarizer 61 is set to 90°, and the initial alignement direction of the liquid crystal molecules 81 is set to 90° (or 0°). Furthermore, the half wave plate 5 is disposed under the upper polarizer 61. In that case, when the electric field for driving the liquid crystal molecules 81 is not applied, the incident light 83 radiated to the TFT-LCD 50 is linearly polarized by the lower polarizer 63, the linearly polarized light does not have phase delays as it passes the liquid crystal molecules 81, and then the polarizing axis of the linearly polarized light is maintained not to pass the upper polarizer 61. However, by the half wave plate 5, the polarizing axis of the linearly polarized light is changed with half wave (λ/2) to pass the upper polarizer 61. Therefore, the IPS mode TFT-LCD can operate in normally white mode. With these features, an additional optical film such as the half wave plate, it is required to increase the manufacturing process and cost.